a) Field of Use
The present invention relates to a brushless motor having a stator core, and, more particularly, to a structure for fixing a stator core and a bearing holder.
b) Description of the Related Art
Examples of conventional circumferentially opposed type brushless motors include those having the kind of structure shown in FIG. 6.
In FIG. 6, the lower end portion of a bearing holder 20 is fixed by press fitting or caulking in the central portion (the left hand end in the drawing) of a base plate 10 made of steel plate or the like.
A spacer 12 is mounted on the base plate 10 around the periphery of the bearing holder 20, and a stator core 48 is mounted on the spacer 12. The stator core 48 comprises a number of core plates stacked one on top of another and fixed together by caulking. The inner periphery of the lowest core plate 14 in the drawing extends toward the center. Screws 18 pass through the base plate 10 and the spacer 12 and are screwed into the core plate 14, and the stator core 48 is thereby fixed to the base plate 10. The stator core 48 has a large number of salient poles disposed radially at its outer periphery. Each of these salient poles has a salient pole bevel portion 49 at its outer end and a drive coil 50 wound around it.
The bearing holder 20 has a cylindrical portion 24, and the outer ring 32 of a ball bearing 30 is press fitted in this cylindrical portion. A spindle 26 is inserted (clearance fitted) in the inner ring 31 of the ball bearing 30. A hub plate 28 is integrally mounted on the end of the spindle 26 which projects upward of the ball bearing 30, and a flat cuplike rotor 42 is mounted on the underside of the hub plate 28. A platelike chucking magnet 40 is mounted on top of the rotor 42. A plate spring 36 is fitted to the underside of the rotor 42, and a drive pin 38 supported by the plate spring 36 passes through a hole 34 in the rotor 42 and projects upward of the upper surface of the hub plate 28.
A drive magnet 44 is fixed to the inner surface of the peripheral wall of the rotor 42 and rotates integrally with the rotor 42. The inner surface of the drive magnet 44 faces the salient pole bevel portions 49 of the stator core 48 across a suitable gap.
The above-described circumferentially opposed type brushless motor having a stator core is constructed so that the rotor 42 can be rotationally driven by detection of rotational position of the drive magnet 44 and by switching of electrical currents through the drive coils 50 in accordance with this detection signal.
Next, another example of a conventional brushless motor, shown in FIG. 7, will be described.
This conventional motor differs from the example shown in FIG. 6 in that the bearing holder 20 has an outer flange portion 62 around its lower portion.
Also, all the core plates constituting the stator core 48 extend radially inward to a position such that their inner peripheral edge surfaces make contact with the outer peripheral surface of the cylindrical portion 24 of the bearing holder 20 and their inner peripheral portions are stacked on the flange portion 62 of the bearing holder 20, and the whole stator core 48 is fixed on the base plate 10 by screws 18 being passed through the base plate 10 and the flange portion 62 and screwed into the stator core 48. The flange portion 62 of the bearing holder 20 is clamped between the stator core 48 and the base plate 10.
This motor also differs from the example shown in FIG. 6 in that the inner surface of the bearing holder 20 comprises a large diameter upper portion and a small diameter lower portion, and a ball bearing 30 is disposed in the large diameter portion and a metal bearing 60 is disposed in the small diameter portion.
Problems Addressed by the Invention
In the conventional brushless motor constructions described above, because the bearing holder is fixed to the base plate by press fitting or caulking, depending on how force is applied to it during the press fitting or caulking process, the shape of the bearing holder can easily be deformed and variation in the internal diameter and height dimensions and inclination of the bearing holder can occur.
Also, in the case where the whole stator core is fixed to the base plate this is done in order to raise the strength of the fastening between the screws and the stator core, but because a lot of space is required where the stator core is mounted on the base plate, it is difficult to secure enough space for the drive pin to descend below the rotor. Furthermore, because the flange portion of the bearing holder outwardly extends far from the rest of the bearing holder, material usage is poor, machining is complicated and this leads to higher cost.